As a method for canceling cross polarization interference in transmission equipment of a co-channel transmission system or the like, an XPIC (Cross polarization interference Canceller) has been used (see Japanese Patent Application Laid-open 2000-165339: this publication will be referred to hereinbelow as patent document 1). This scheme performs cancellation of cross polarization interference by generating a signal that cancels the interference signal from an orthogonal polarization wave (which will be referred to hereinbelow as opposite polarization) relative to an observable polarization wave (which will be referred to hereinbelow as main polarization) with reference to the opposite polarization-received signal and by adding it to the received signal.
In order to operate the XPIC, it is necessary to synchronize the interference wave with the carrier component of the main polarization received signal. In a quasi-coherent detection system, the reception local synchronization scheme for performing synchronization with a local signal on the receiver side is preferable. In the case of the reception local synchronization scheme, the local signal on the transmitter side does not need to be synchronized.
As a method for realizing reception local synchronization when using an XPIC in the co-channel transmission system, there are the common local scheme and the reference synchronization scheme. The common local scheme branches the output from a single RF local oscillator into two polarizations and supplies them to receivers for individual polarized waves. The reference synchronization scheme supplies the output from a low-frequency reference oscillator to each separate RF local oscillator (LO: Local Oscillator) in the receiver for each polarization, so that each local oscillator generates a RF local signal in synchronization with the reference oscillator to thereby perform synchronization with the local signal for each polarization.
Since, in the common local scheme, the output from a single RF local oscillator is branched into two parts to be supplied to different polarization receivers, the phase noise of the RF local signal does not affect the XPIC characteristics. However, if the RF local oscillator breaks down, the communications through both polarizations shut down, hence it is disadvantageous in terms of communications path reliability.
On the other hand, in the reference synchronization scheme, since each polarization receiver includes its own local oscillator, if one of them breaks down the transmission path of the other polarization that has not broken down and that will not shut down. Hence this method is advantageous in terms of communications path reliability. However, in this method, the phase noises from the RF local oscillators cause degradation of the characteristics of cross polarization interference cancellation. For this reason, RF local oscillators low in phase noise are used, but such oscillators low in phase noise are expensive, hence this scheme is disadvantageous in view of cost.
A conventional cross polarization interference canceling method will be described.
FIG. 1 is a diagram showing a configurational example of a cochannel transmission system using a common local scheme. IF (Intermediate Frequency) signals transmitted using V (vertical)-polarization and H (horizontal)-polarization are converted by means of mixers 1, 1′ and oscillators 2, 2′ into RF signals, which are sent out from antennas 3, 3′. The transmitted signals are received by reception antennas 4, 4′ on the receiver side. Here, for description convenience, pairs of antennas 3, 3′ and 4, 4′ are shown separately so as to correspond to individual polarizations. In reality, however, 3, 3′ and 4, 4′ are each made of a single antenna.
In the common local scheme in FIG. 1, the output from single local oscillator 6 is branched and used as the RF local signals for converting RF signals into the IF signals.
FIG. 2 is a diagram showing an interior configurational example of local oscillator 6. Local oscillator 6 includes reference oscillator 14 that outputs a low-frequency signal as a reference, phase comparator (PD: Phase Detector) 15, voltage controlled oscillator 16 and frequency divider 17. Input to phase comparator 15 are the output from reference oscillator 14 and the signal that was obtained by frequency-dividing the output from voltage controlled oscillator 16 through frequency divider 17. Since the output from phase comparator 15 is input to voltage controlled oscillator 16, voltage controlled oscillator 16 constitutes a PLL (Phased Locked Loop) that oscillates at n-times the frequency of reference local oscillator 14. The output from voltage controlled oscillator 16 is used as the RF local signal.
In FIG. 1, the RF signal input to the receiver through antenna 4,4′ is converted to the IF signal through mixer 5,5′ and input to orthogonal demodulator 8,8′. The signal input to orthogonal demodulator 8,8′ is orthogonally demodulated by local oscillator 7,7′, then the orthogonally demodulated signal is input to DEM (demodulator) 9, 9′, where the signal is processed by carrier reproduction, clock reproduction, and the like. The demodulated result is output as the main signal to adder 10,10′.
On the other hand, the IF signal input from the opposite polarization side is also input to orthogonal demodulator 11,11′, so that the orthogonally demodulated signal is input to cross polarization interference canceller (which will be referred to hereinbelow as XPIC) 12, 12′. In XPIC 12, 12′, the opposite polarization signal that has interfered with the main polarization signal by cross polarization interference arising through the transmission path is detected to generate and output a signal that cancels it. The signal output from XPIC 12, 12′ is adjusted as to its phase rotation to that on the main signal side by EPS (Endless Phase Shifter) 13, 13′, and the resultant is added to the main signal at adder 10, 10′ to thereby compensate cross polarization interference.
FIG. 3 is a circuit block diagram showing in detail one constructional example of the part downstream of the outputs from orthogonal demodulators 8 and 11. In FIG. 3, complex multiplier 18, carrier phase comparator (Carr PD) 20, loop filter (Carr LPF) 21, accumulator (Acc) 22 and SIN/COS table 23 constitute a carrier reproduction PLL, and this loop reproduces the carrier.
In XPIC 24, if there is an opposite polarization input, a signal that cancels the cross polarization interference component that interfered with the main polarization is generated. In order to match the carrier phase of the interference wave mixed in the main polarization with the carrier phase of the compensating signal output from XPIC 24, complex multiplier 18′ rotates the output signal from XPIC 24 by the same angle as the rotational angle of the main polarization. The output from complex multiplier 18′ is added to the main polarization at adder 19 so as to cancel cross polarization interference. Complex multiplier 18′ corresponds to a phase rotator.
Since in the case of this common local scheme, the local signal used in each polarization receiver is supplied from common local oscillator 6, the V-polarization that has been affected by phase noise φ1, namely signal V(φ1) and the H-polarization signal that was mixed in due to cross polarization interference and affected by phase noise φ1, namely signal H(φ1) are input to the V-polarization receiver. Further, as to the opposite polarization input, the H-polarization signal that has been affected by phase noise φ1 is applied in the form of H(φ1). Here, as to the relationship between the carrier component of the opposite polarization signal that has interfered with the main polarization and the carrier component of the received signal of the opposite polarization signal, they are totally identical as to both frequency and phase because the same output from local oscillator 6 is used for processing.
The phase noise component of the opposite polarization component mixed into the main polarization is φ1 while the phase noise of the received signal on the opposite polarization side is also φ1, hence there is no phase difference between the two signals due to phase noise. XPIC 12 shown in FIG. 1 can generate a phase-stable correcting signal without having any influence from phase noise when generating a correcting signal. That is, in this scheme, the phase noise from the local oscillator will not affect the capacity of cross polarization interference cancellation.
However, as stated above the problem entailing the common local scheme is that communications via both the polarizations shut down all at once if local oscillator 6 breaks down because the output from this single local oscillator 6 is branched for use. This feature is disadvantageous in terms of securing communications path reliability.
FIG. 4 is a diagram showing a configurational example of a cochannel transmission system adopting a reference synchronization scheme. As shown in FIG. 4, this configuration is the same as that of the common local scheme in FIG. 1 except that each polarization receiver includes local oscillator 6 or 6′. In the reference synchronization scheme, each polarization receiver includes local oscillator 6, 6′. Then, in order to synchronize the frequencies of local oscillators 6, 6′ with each other, the output of a reference signal from low-frequency reference oscillator 25 that is to be the reference is branched so that each local oscillator 6, 6′ can generate a local signal synchronized with the reference signal. With this architecture, if one of local oscillators 6, 6′ has broken down, the transmission path via the other polarization which is not broken will not be cut off.
FIG. 5 is a diagram showing a configurational example of local oscillator 6, 6′ used in the reference synchronization scheme. As shown in FIG. 5, local oscillator 6, 6′ includes phase comparator 15, voltage controlled oscillator 16 and frequency divider 17. Input to phase comparator 15 are a low-frequency reference signal from without and the signal which is the output from voltage controlled oscillator 16 that is n-th frequency-divided by frequency divider 17. Since the output from phase comparator 15 is input to voltage controlled oscillator 16, voltage controlled oscillator 16 constitutes a PLL that oscillates at n-times the frequency of the reference signal input. The output from voltage controlled oscillator 16 is used as the RF local signal. In the reference synchronization scheme, the local signals used for the polarizations are generated by different PLLs, so that the phase noises arising have no correlation with the other.
Accordingly, in the reference synchronization scheme, due to the phase noises involved with separate local oscillators 6, 6′ of the V/H polarization receivers, the phase relationship between the local signals output from these local oscillators 6 and 6′ is always changing. As a result, a phase change that reflects the difference between the phase noises of local oscillators 6 and 6′ appears between the phase of the opposite polarization component that was mixed into the main polarization and the phase of the received signal on the opposite polarization side. More specifically, as shown in FIG. 4, when the signal in the V-polarization reception line that was affected by phase noise φ1 arising in local oscillator 6 is expressed as V(φ1), the component from the H-polarization that has interfered with the V-polarization in the transmission path is affected by phase noise φ1 that arises in local oscillator 6, forming H(φ1). Resultantly, a signal V(φ1)+H(φ1) as the result of cross polarization interference is input to the V-polarization reception line.
On the other hand, in XPIC 12 a H-polarized received signal named H(φ2) that has been affected by phase noise φ2 of local oscillator 6′ from the H-polarization reception line is input. Here, for simplicity the interference with the H-polarization from the V-polarization will not be considered. XPIC 12 generates a signal that cancels out H(φ1) that interfered with the main polarization input by reference to the opposite polarization input named H(φ2). At the same time, XPIC 12 has to generate a correction signal by taking into consideration even the phase difference (φ1−φ2). Since an XPIC generally has a phase rotating function, it is possible to perform correction following the phase difference (φ1−φ2) originated from phase noise when the temporal change of this phase difference is slower than the time constant of the XPIC operation. However, if phase change exceeding the time constant of the XPIC occurs due to a phase noise, it is impossible for the XPIC to achieve proper compensation, then characteristics degradation appears.
This behavior will be described with FIG. 6. FIG. 6 is a chart showing the behavior of the phase noise in a RF local signal, representing a spectrum centered at the oscillation frequency of the local signal. This shows that the farther away the frequency moves from the center, the lower the power density becomes, and that the father away the frequency moves from the center, the smaller the frequency component of the phase noise becomes. The phase noise component in the limited low-frequency area can be followed by the XPIC and the influence of the phase noise can be compensated.
However, the XPIC cannot react in the region beyond the range which is possible for the XPIC to follow the phase noise, so that the phase noise component is output directly from the XPIC. As a result, when the XPIC output is added to the main polarization signal, it will not match the phase of the interference wave in the main polarization, hence producing compensation error, resulting in characteristic deterioration. It is possible to increase the speed for following the phase noise if the time constant of updating XPIC tap coefficients is made greater. However, this increases the noise arising from the XPIC itself, so in effect there is a limit to increase the time constant of the XPIC. For this reason, when an XPIC based on a reference synchronization scheme is used, it is necessary to use an expensive local oscillator that provides low phase noise figure.